In the production of oxygen or any other gas, it is desirable to improve the performance of the PSA process and reduce operating and capital costs. One way to achieve this is to reduce the amount of adsorbent and/or reduce power consumption while maintaining high product recovery. Reducing the adsorbent particle size to improve the adsorption rate within the particle is often cited in the related art. More recently, the small particle strategy has been combined with advanced adsorbents having high intrinsic pore diffusivity to improve separation processes. Such strategies must contend with the effects of increasing pressure drop with decreasing particle size. Finally, many attempts have been made to utilize rapid pressure swing adsorption (RPSA). RPSA is characterized by relatively high pressure drop, long (deep) beds and relatively low product recovery.
U.S. Pat. No. 5,891,218 (Rouge et al.) discloses a relationship between particle size and bed thickness for optimizing the cost of a PSA air separation process. Preferred adsorbents include both Type X and Type A zeolite adsorbents for air separation. The preferred range of bed thicknesses (e.g., 0.1 m to 2.0 m) and mean particle diameter (e.g., 0.1 mm to 5.0 mm) must be selected according to the following rule: a PSA process, such that300≦L/√{square root over (dp)}≦1000,with particle diameter (dp, measured in mm) and bed thickness (L, measured in mm). Process cycle times in the range of 45 s to 90 s are disclosed.
U.S. Pat. No. 5,122,164 (Hirooka et al.) describes 6, 8 and 10-step vacuum pressure swing adsorption (VPSA) processes for separating air to produce oxygen. While the main thrust of this patent is the cycle configuration and detailed operation of the various cycle steps to improve yield and productivity, Hirooka et al. utilize small particles to achieve faster cycles. A broad particle range is specified (8×35 US mesh or 0.5 mm to 2.38 mm), but 12×20 US mesh or 0.8 mm to 1.7 mm is preferred. Half-cycle times of 25 s to 30 s are indicated (total cycle times of 50 s to 60 s).
Wankat (Ind. Eng. Chem. Res., 26(8), pp. 1579-1585, 1987) and Rota et al. (AIChE J., 36(9), pp. 1299-1311, 1990) describe the concept of “intensification” whereby decreased particle diameter is employed to produce shorter columns and faster cycles. By non-dimensionalizing the governing mass balance equations for the adsorption process, a set of scaling rules are suggested that preserve the performance of the process in terms of product recovery, purity and pressure drop while increasing the adsorbent productivity. These theoretical results are based upon the similarity of dynamic adsorption behavior. The similarity concept presumes an idealized constant pattern mass transfer front, with the length of the mass transfer zone (LMTZ) directly proportional to the square of the particle diameter when pore diffusion is controlling.
U.S. Pat. No. 6,500,234 (Ackley et al.) describes processes utilizing advanced adsorbents with high intrinsic diffusivities relative to conventional adsorbents. Increased oxygen product recovery was demonstrated by increasing the rates of adsorption/desorption to create higher nitrogen mass transfer coefficients at a fixed pressure ratio. This concept was then applied to achieve very short cycles (e.g., greater than 10 s) and very low bed size factor (BSF) while affecting only minimal decrease in product recovery. Particle diameters of 0.5 mm and larger were considered.
U.S. Pat. No. 6,551,384 (Ackley and Zhong) extends the concepts of U.S. Pat. No. 6,500,234 to small scale medical oxygen concentrators by combining small particles and short beds with high intrinsic diffusivity. Short cycles as low as 4 s were demonstrated to achieve BSF as low as 50 lb/TPDO utilizing a mean particle diameter of 0.55 mm.
Very small adsorbent particles (0.1 mm to 0.8 mm) are necessary for the fast cycles and high specific pressure drop that characterize a special class of processes known as rapid pressure swing adsorption (RPSA). Typical RPSA processes have very short feed steps operating at high feed velocities, include a flow suspension step following the feed step and generally have total cycle times less than 20 s (often less than 10 s). The behavior of the adsorption step is far removed from that in conventional processes wherein the state of the bed is dominated by an equilibrium zone with the remaining small bed length fraction occupied by the mass transfer zone (MTZ), i.e., as determined at the end of adsorption. In contrast, the working portion of the bed in RPSA is primarily mass transfer zone with only a relatively small fraction of the bed operating in equilibrium. The high pressure drop/short cycle combination (wherein the pressure drop is on the order of 12 psi/ft) is necessary to establish an optimum permeability and internal purging of the bed which operates continuously to generate product.
RPSA is clearly a special and distinct class of adsorption processes known in the art. The most distinguishing features of RPSA compared to conventional PSA can be described with respect to air separation for oxygen production. The pressure drop per unit bed length (ΔP/L) is at least an order of magnitude greater and the particle diameter (dp) of the adsorbent is usually less than 0.5 mm in RPSA. Total cycle times are typically shorter and the process steps are different in RPSA. Of these contrasting features, pressure drop and particle size constitute the major differences.
Alpay et al. (Chem. Eng. Sci., 49(18), pp. 3059-3075, 1994) studied the effects of feed pressure, cycle time, feed step time/cycle time ratio and product delivery rate in RPSA air separation for several ranges of particle sizes (e.g., 0.15 mm to 0.71 mm) of 5 A molecular sieve. A relatively long bed (e.g., 1.0 m) with high pressure drop was employed as required for RPSA. Alpay found maximum separation effectiveness (maximum oxygen purity and adsorbent productivity) for particles in the size range 0.2 mm to 0.4 mm.
R. H. Kaplan et al. (AIChE Annual Meeting, Nov. 7, 1989, San Francisco) traced developments in the design of concentrators and selected a RPSA system. Using a three-bed system (bed length of 406 mm) and small adsorbent particles (40×80 beads), the cycle time was reduced to as low as 2.4 s. The oxygen recovery was only about 25%, and the BSF was estimated to be about 200 lbs/tons per day of oxygen (TPDO) when operating at an adsorption pressure of 30 psig and a pressure ratio of about three. Such a recovery is relatively low compared to conventional VPSA processes that achieve oxygen recovery in the range of 50% to 70%. The pressure drop in this RPSA system was large, about 8 psi/ft at 0.3 m/s superficial velocity compared with 0.1 psi/ft in conventional large oxygen PSA units. The low recovery and high pressure drop result in a concentrator having relatively high power consumption.
Three critical factors of PSA process performance are: (1) adsorbent amount (or bed size factor (BSF) given as pounds of adsorbent per ton of product oxygen produced per day (lb/TPDO); (2) oxygen recovery; and (3) unit power consumption (kW/TPDO). Fast cycles are a path to reduce bed size which affects both the adsorbent and vessel costs. However, such a strategy is also accompanied by a tendency toward lower product recovery, higher pressure drop and increased power consumption. In the past, attempts to solve this problem included simply decreasing particle size (U.S. Pat. No. 5,122,164), intensification (Wankat et al.) or adsorbent rate enhancement (U.S. Pat. No. 6,500,234 and U.S. Pat. No. 6,551,384). All of these methods suffer from higher pressure drop. A somewhat different approach is represented by RPSA. Although very short cycles are employed with small particles and relatively long beds, RPSA processes are driven by high pressure drop and are generally characterized by low product recovery and high power consumption.
As discussed herein, the present invention avoids or minimizes the limitations of the previously known processes by first recognizing a change in the adsorption rate-limiting mechanism from pore diffusion to axial dispersion as a function of adsorbent particle size and process operating conditions. The objectives of the present invention are satisfied by selecting the proper combination of particle size, bed depth and cycle time.